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Date: Fri, 8 Feb 2013 08:17:32 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Michel Lespinasse <walken@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>, Rik van Riel <riel@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, David Howells <dhowells@redhat.com>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        linux-kernel@vger.kernel.org, john.stultz@linaro.org
Subject: Re: [RFC PATCH 1/6] kernel: implement queue spinlock API
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On Thu, Feb 07, 2013 at 09:11:14PM -0800, Michel Lespinasse wrote:
> On Thu, Feb 7, 2013 at 9:03 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > Right...  For spinlocks that -don't- disable irqs, you need to deal with
> > the possibility that a CPU gets interrupted while spinning, and the
> > interrupt handler also tries to acquire a queued lock.  One way to deal
> > with this is to have a node per CPUxirq.  Of course, if interrupts
> > handlers always disable irqs when acquiring a spinlock, then you only
> > need CPUx2.
> 
> The simple solution would be to do like I proposed in my faster queue
> spinlock proposal, have one function for process context lock
> acquisitions, another for bh-disabled acquisitions, and just say that
> hardirqs can't use the queue spinlocks (I don't expect we have any
> locks taken from hardirq context where contention might be an issue ?)

Makes sense!  The spinlocks that disable hardirqs should get extra
contention-reduction attention, after which the main benefit for
queued spinlocks is process-level spinlocks.

							Thanx, Paul